Deep evolutionary origin of gamete-directed zygote activation by KNOX/BELL transcription factors in green plants

KNOX and BELL transcription factors regulate distinct steps of diploid development in the green lineages. In the green alga Chlamydomonas reinhardtii, KNOX and BELL proteins are inherited by gametes of the opposite mating types, and heterodimerize in zygotes to activate diploid development. By contrast, in land plants such as Physcomitrella and Arabidopsis, KNOX and BELL proteins function in meristem maintenance and organogenesis during the later stages of diploid development. However, whether the contrasting functions of KNOX and BELL were acquired independently in algae and land plants is currently unknown. Here we show that in the basal land plant species Marchantia polymorpha, gamete-expressed KNOX and BELL are required to initiate zygotic development by promoting nuclear fusion in a manner strikingly similar to that of C. reinhardtii. Our results indicate that zygote activation is the ancestral role of KNOX/BELL transcription factors, which shifted toward meristem maintenance as land plants evolved.